1. Field of the Invention
This invention relates to a spark plug for an internal combustion engine in which a noble metal firing tip is secured to a front end of a center electrode.
2. Description of Prior Art
In a center electrode of a spark plug for an internal combustion engine, a composite structure has been used in which a heat-conductive core (Cu) is embedded in a heat-and erosion-resistant clad metal (nickel-based alloy) as shown in Japanese Patent Publication No. 59-2152. According to the Japanese Patent Publication No. 59-2152, a nobel firing tip is further bonded to a front end of the clad metal by means of electric resistance welding so as to improve its spark-erosion resistant property. After completing the electric resistance welding, the firing tip and the front end of the clad metal are milled respectively to make them diametrically even.
In another Japanese Patent Publication No. 63-57919, a hole is provided at a front end surface of the clad metal, and the firing tip is placed within the hole to apply a laser beam welding from the front end of the clad metal to the firing tip.
In the former prior art, the electric resistance welding causes to heat and press the firing tip so as to make the edged corner of the firing tip rounded. As a result, a higher voltage is required for the spark plug to discharge between its electrodes. In order to recover the original edged corner, it is necessary to mill the firing tip only to waste the expensive noble metal.
In the latter prior art, it is necessary at the time of the laser beam welding to place the firing tip deep enough into the hole to positively secure the firing tip against inadvertent removal. This requires an increased quantity of the noble metal to make the firing tip costly.
Therefore, it is one of the objects of the invention to provide a spark plug in which a noble metal firing tip is positively bonded to a front end of a center electrode by means of a laser beam welding without sacrificing a good ignitability with a minimum amount of noble metal, and thus contributing to an extended service life with relatively low cost.